Fiber optic digital data transmitting system

ABSTRACT

A fiber optic digital data transmitting system is disclosed which has the capability of transmitting and accurately reproducing digital data signals at the receiver even when the optical signal is attenuated in the fiber optic transmitting medium. A composite signal is produced at the transmitter which is the time coincident sum of the non-zero amplitude of the digital data signal to be transmitted and a time varying signal which encodes each non-zero amplitude of the digital signal and other information. The composite signal modulates an optical carrier signal which is coupled to a fiber optic transmission medium which couples the transmitter to the receiver. At the receiver, the presence of each time varying signal is detected as a non-zero amplitude of the digital signal. Circuitry is provided in the receiver for producing a pulse in response to the detection of each time varying signal for reproducing the transmitted digital signal and for detecting any information in addition to the non-zero amplitude of the digital signal which has been encoded in the time varying signal. The present invention is compatible with existing PCM systems which utilize threshold detection.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of Ser. No. 288,111, filed July 29,1981, entitled "Fiber Optic Digital Data Transmitting System," now U.S.Pat. No. 4,420,842.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to digital data transmission systems and to fiberoptic digital data transmission systems.

2. Description of the Prior Art

Fiber optic digital transmission systems have been developed which havetremendous capacity to transmit large amounts of information over asingle channel because of the large bandwidth which may be modulatedupon an optical carrier signal. While the bandwidth of fiber optictransmission systems is a tremendous advantage in comparison to wire orradio frequency transmission mediums, digital fiber optic communicationsystems are nevertheless subject to problems caused by signalattenuation during transmission and noise.

The simplest form of detecting digital fiber optic communications is byusing a threshold detector which senses every signal above a giventhreshold as a bit of information and every signal below the thresholdas the absence of a bit. Threshold detectors are not able to reliablydistinguish between high amplitude noise pulses and the transmitteddigital data. Moreover, even though the optically encoded digital signalwhich is coupled to the fiber optic transmission medium from atransmitter or repeater may be relatively free from high amplitudenoise, the fiber optic transmission medium may attenuate the digitalsignal sufficiently during transmission to drop the digital signalamplitude down close to or below the threshold level. For example, it isknown that attenuations of the digital signal level in a fiber optictransmission medium of up to 20 dB or more may occur as a consequence of"poor" quality splices which may be caused by a myriad of factors. Tominimize digital signal attenuation in fiber optic transmission mediums,expensive connectors are used which require skilled labor to install.Moreover, expensive low noise threshold detectors are used to permitdetection of attenuated digital signals. The difficulty in detectingattenuated digital signals transmitted on fiber optic transmissionmediums requires the use of a larger number of repeaters to amplify thedigital signal amplitude than otherwise would be required if thresholddetectors could reliably detect low signal to noise ratio signals.

Current fiber optic communication systems use light emitting diodeswhich are operated at high power levels to produce a digital signalwhich has been modulated to a high amplitude. The operation of the lightemitting diodes at high power levels can cause their premature failure.The operation of the light emitting diodes at high power levels is aconsequence of the requirement that the amplitude level of the opticalsignal must be boosted to a sufficiently high level to permit accuratethreshold detection and increased distance between repeaters.

In order to avoid the inherent unreliability of threshold detectors inaccurately detecting the digital data which is being transmitted by afiber optic transmission medium, sequenced voting devices have beenused. In this approach, a digital signal is broken up into a number ofslices, 16 for example, which are each "voted on" by single level ormultilevel threshold detectors. If a certain number of thresholddetectors affirm the presence of a digital signal, then a fixed digitalsignal is regenerated. The disadvantage of the sequenced voting deviceshas been their expense, and they are highly data-speed sensitive.

Satellite communications systems are extremely susceptible to problemscaused by inaccurate detection of digital data at a transmitting stationprior to transmission. Because of the approximate one-third of a secondrequired to communicate between two ground stations via a relay from ageosynchronously orbiting satellite, any error in detecting a digitalsignal at a ground transmitting system, which is to be relayed toanother ground station via satellite, after transmission will presentthe satellite communication system with a difficult error correctionproblem. To date, correction of detection errors which are discoveredafter transmission by a ground transmitting system have required thebuffering of large amounts of data and sophisticated data processingbecause of the extremely high transmission bit rates which arecharacteristically used by current satellite multiplex communicationsystems. In the future when the number of ground stations, satellitesand data rates are projected to dramatically increase for digitalsatellite communication systems, the requirement of accurately detectingdata at ground stations prior to transmission will be even more acutebecause of the projected increase in information being transmitted. Adata transmission system which economically detects and reduces the rateof transmission of erroneous data will lessen the amount of bufferingand data processing equipment required to correct erroneous digital datatransmission below that needed in current satellite systems. Since thereis a tremendous advantage to communicating with ground stations via afiber optic transmission medium because of bandwidth and costconsiderations, a highly accurate detector for detecting fiber opticallytransmitted data at a ground station would be of great use in improvingcommunication.

Currently, methods used to assist in error correcting are becomingprogressively complex and expensive. One major network technique, timedivision multiplexing, allows separation of channels by an interval oftime, but as data rates increase than not only are more accurate clocksnecessary to determine accurate time intervals but accurate timesynchronization between different points of a network can becomeoverwhelmingly burdensome and failure prone. Another network techniquesthat is tending to become burdensome in the existing prior art is theuse of parity bits and/or address information bits that proceed orfollow a stream/package of data bits. Parity bits in conjunction withprotocol bits are now burdening data streams (especially when there maybe hundreds of data initiating devices in a network) with overhead datathat is mounting to 20% and even to 40% of the data being transmitted.All this overhead must be handled, rehandled and separated from theactual data.

Frequency shift keying is a known modulation technique for transmittingdigital data which uses two discrete frequencies to encode the high andlow levels of a digital signal. The signal format of frequency shiftkeying does not transmit a fixed amplitude component representative ofthe high level bit position and additional information such as thepresent invention. Systems using frequency shift keying are notcompatible with existing digital data transmitting systems which detectPCM by threshold detection.

SUMMARY OF THE INVENTION

The present invention is a digital communication system which in itspreferred embodiment is a digital fiber optic communication system. Thefiber optic communication system of the present invention has thecapability of transmitting and accurately detecting the digital datasignal at a receiver even when the optical signal is attenuated duringtransmission. The capability of the present invention to accuratelydetect digitally transmitted data at the receiver reduces the need forhigh cost splicings in the fiber optic transmission medium which requiretime and skilled labor to implement, "low" noise detectors and closespacing between repeaters to insure that the signal level is notattenuated below a level at which accurate detection can be made.

In accordance with the invention a composite signal is produced at thetransmitter which is the time coincident sum of the non-zero amplitudesof the digital data signal to be transmitted and a time varying signalwhich encodes at least each non-zero amplitude of the digital signal.The composite signal modulates an optical carrier signal which iscoupled to a fiber optic transmission medium. The time varying signalmay be but is not limited to a single burst of constant frequencyalternating current. The time varying signal may be of any known analogor digital encoding format in view of a great bandwidth available in afiber optic communications system.

The receiver is designed to respond to the frequency or frequencies ofthe time varying signal which are contained within the modulated opticalcarrier wave. At the receiver, the detection of each time varying signalindicates the reception of a non-zero amplitude of the digital signal.Means are provided in the receiver for producing a pulse in response tothe detection of each time varying signal for reproducing thetransmitted digital signal and for detecting any information in additionto the presence of a non-zero amplitude which is encoded in the timevarying signal.

The transmission of the time varying signal in time coincidence with thenon-zero amplitudes of the digital signal has advantages. By designingthe receiver to respond to a narrow frequency range surrounding thefundamental frequency or frequencies of the time varying signal, theability to detect low amplitude digital signals may be improved overthat possible with threshold detectors. In addition, the detection ofthe fundamental frequency or frequencies of the time varying signal maybe implemented by using conventional electronic components withoutrequiring data processing. The present invention is compatible withexisting fiber optic digital transmission systems which are not designedto respond to the time varying component of the composite signal.Conventional threshold or "voting" devices may be used to detect thedigital part of the composite signal without any interference from thetime varying component which may be ignored.

In view of the tremendously wide bandwidth which is present in fiberoptic digital communication systems, the time varying signal componentof the composite signal may be used to transmit large amounts ofadditional information which could be used to communicate information tothe receiver which facilitates error detection, the identification ofthe transmitter, or establishes a priority of communication betweendifferent parts of the system in accordance with known multi-stationdata communication techniques. It should be understood that there is nolimitation to the type of information which may be transmitted in thetime varying signal component of the composite signal.

The following are definitations of terms used throughout thespecification. A composite signal is the sum of the non-zero amplitudesof the digital signal to be transmitted and a time varying signal whichis any signal which changes amplitude over the time interval that anon-zero amplitude of the digital signal is present. Time coincidentdefines a time interval during which non-zero amplitudes of the digitalsignal and the time varying signal are simultaneously present. The timevarying signal may periodically drop to zero amplitude during eachnon-zero amplitude of the digital signal and be described as timecoincident. Optical carrier signal is any bandwidth of electromagneticradiation which may be transmitted by an optic transmission medium.Optical transmission medium is any guided or unguided physical mediumfor conveying an optical carrier signal including fiber optics,integrated optics and atmospheric and space medium.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a first embodiment of a fiber optic digitalcommunication system in accordance with the present invention which usesa light emitting diode as an internally modulated source of opticalcarrier signal;

FIG. 2 illustrates a second embodiment of a fiber optic digitalcommunication system in accordance with the present invention which usesa laser as a continuous wave source of an optical carrier signal withexternally modulated continuous wave beams;

FIG. 3 illustrates examples of the composite signal which may be usedwith the present invention; and

FIG. 4 illustrates a time varying signal detector which may be used todetect the transmitted composite signal.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates a first embodiment 10 of the present invention whichuses an optical carrier wave emitted by a light emitting diode totransmit digital data on a fiber optic transmission medium 12 between atransmitter 14 and a receiver 16.

A transmitter in accordance with the first embodiment is adapted to becoupled to a source of digital data 18 which is to be transmitted. Thesource of data may be a computer or any other source which producesbinary data having a zero amplitude level and a non-zero amplitude levelto encode two signal states in accordance with conventional encodingtechniques. The source of digital data is coupled to a time varyingsignal generator 20 which in this embodiment produces a burst ofoscillations of a duration no longer than the non-zero amplitude of theindividual data bits which are coupled to the time varying signalgenerator. The frequency of oscillation of the time varying signalgenerator is chosen to be sufficiently high that several cycles ofoscillation will be completed during the period of time coincidencebetween each non-zero amplitude of the digital signal and the timevarying signal which is used to encode at least the presence of anon-zero amplitude of the digital signal. The output of the time varyingsignal generator 20 is coupled to an amplifier 22 which sums the keyedoscillations produced by the time varying signal generator with thenon-zero amplitudes of the digital data signal. The level of thecomposite signal at all points in time is preferably chosen to be abovethe threshold level of detection of existing PCM detection systems. Theoutput composite signal 24 of the amplifier is coupled to light emittingdiode 26 which produces an optical carrier signal which has beenmodulated in intensity in accordance with the composite signal. Themodulated optical carrier is coupled to a fiber optic transmissionmedium 12 which couples the transmitter 14 to the receiver 16. It shouldbe understood that each of the elements used in the transmitter is ofconventional design.

The receiver 16 functions to detect the modulated optical carrier wavewhich is transmitted by the fiber optic transmission medium 12. Thedetection of the modulated optical carrier receiver includes a lightdetector 30 which may be p.i.n. photo-detector, photo-transistor,avalanche photo-diode, avalanche reach-through, photo-diode,photo-multiplier tube, or any other device which produces an outputvoltage in response to variations in the intensity of the modulatedoptical carrier wave. The output signal from the light detector 30 iscoupled to an amplifier 32 which has a linear amplificationcharacteristic for producing an output signal of sufficient gain topermit detection of the time varying signal component of the compositesignal. The output of the amplifier 32 is coupled to a time varyingsignal detector 34 which functions as a phase lock loop. The timevarying signal detector 34 may be a Signetics NE 560 chip which willrespond to tones varying in frequency from approximately 1 Hz to 15 Hzor an EXAR S 200 which responds to frequencies up to 30 M Hz. The timevarying signal detector 34 resonates in response to the fundamentalfrequency or frequencies of the tones produced by the time varyingsignal generator 20. The time varying signal detector 34 has twooutputs, the first 36 being for information other than the detecteddigital signal and the second 38 being a pulse train corresponding tothe transmitted digital data. The second output 38 is coupled to a pulseforming circuit 39 such as a one shot multivibrator which converts theoutput of the time varying signal generator into a series of pulseshaving a fast rise time. The one shot multivibrator may be a TexasInstruments LS 221. The pulses from pulse forming circuit 39 may beprocessed by data processing equipment, etc. Detection of a received bitmay be enhanced even though the overall amplitude of the compositesignal has been attenuated to a level to make threshold detectiondifficult. Significant enhancement of detection would occur where thenoise distribution decreases, typically at higher frequencies than thedata rate. In addition, the time varying signal generator 20 may containother conventional detection circuitry which is designed to respond toany format of encoding used at the transmitter by the time varyingsignal generator 20. Information contained within the time varyingcomponent of the composite signal could be used for identification ofthe transmitter, error checking or establishing the priority of thetransmitter in the overall communication system, etc.

FIG. 2 illustrates a second embodiment 40 of the present invention. Likeparts in FIGS. 1 and 2 are identified by identical reference numerals.The receivers 16 of FIGS. 1 and 2 are identical. A continuous wave laser42 is used as the source of the optical carrier wave. The output beam oflight 44, which is produced by laser 42, is imaged upon an opticalmodulator 44 which preferably is a STARK, POCKELS or BRAGG cell but isnot limited thereto. The optical modular 46 produces an output beam oflight which is imaged upon the fiber optic transmission medium 12 totransmit the composite signal which is produced by the source of digitaldata 18, time varying signal generator 20 and summing amplifier 22 inthe identical manner described in conjunction with FIG. 1 supra. Theoutput of the summing amplifier 22 is coupled to the optical modulator44 to modulate the optical carrier wave produced by laser 40 inaccordance with the composite output signal produced by summingamplifier 22.

The present invention is not limited to the type of additionalinformation which is transmitted in the time varying signal nor the typeof time varying signal that is used. For example, while the embodimentsof FIGS. 1 and 2 use a single frequency tone to encode the presence of anon-zero amplitude in the digital signal without conveying additionalinformation, the frequency of the tone in FIGS. 1 and 2 could alsoencode the identity of the transmitter by assigning a unique frequencyto each transmitter. The time varying signal generator 20 which is usedto encode any additional information could be any analog or digitalsignal generator which is activated in response to the presence of eachnon-zero amplitude in the digital signal which is to be transmitted.Specifically, but not limited thereto, the time varying signal may be adigital signal of any known format, a single frequency burst presentthroughout the duration of each non-zero level of the digital signal, aseries of single frequency bursts which are separated by zero amplitudeintervals during the duration of each non-zero amplitude level of thedigital signal to be transmitted, a series of bursts of differentfrequency which are separated by zero amplitude intervals during theduration of each non-zero amplitude level of the digital signal to betransmitted. If the time varying component contains digital information,the time varying signal detector 34 may be programmed to recognizecertain patterns of digital information which can be discriminated evenwhen the composite signal is attenuated. The nature of the time varyingsignal which is generated at the transmitter 14 dictates the design ofthe time varying signal detector of the receiver 16. For each type oftime varying signal generator 20 at the transmitter 14 there will be acorresponding time varying signal detector 34 at the receiver 14 whichis designed to detect the digital data and all other information whichis encoded in the time varying signal.

FIGS. 3(a), 3(b), 3(c), and 3(d) illustrate examples of composite signalformats which may be used with the present invention. FIG. 3(a)illustrates a composite signal 24 having a high level component 50representative of a high level bit and a single frequency sinusoidaltone 52 which is present for the entire duration of the high levelsignal. The frequency of the tone 52 may be chosen to encodeinformation. FIG. 3(b) illustrates a composite signal 24 having a highlevel component 50 and a plurality of single frequency sinusoidal tones54. The number and pattern of the tones 54 are used to encode additionalinformation. FIG. 3(c) illustrates a composite signal 24 having a highlevel component 50 and a plurality of tones 56, each having a differentfrequency. The tones 56 are used to encode additional information. Thecomposite signal 24 represented by FIGS. 3(a), 3(b), and 3(c) may begenerated by standard frequency synthesis. The time varying signalgenerator 20 may use a programmed EXAR S 200 chip which in conjunctionwith 74 LS293 counters may be programmed to produce frequencies up to 30MHz. FIG. 3(d) illustrates a composite signal having a high levelcomponent 50 and a PCM signal 58. The PCM signal 58 is used to transmitadditional information. The composite signal 24 represented by FIG. 3(d)may be generated by the time varying signal generator 20 which may beproduced by squaring the output from the frequency snythesizer describedfor producing the modulation of FIG. 3(c).

FIG. 4 illustrates a time varying signal detector 34 which may be usedin the embodiments of FIGS. 1 and 2. The input signal 63 to FIG. 4 isproduced by the amplifier 32 of FIGS. 1 and 2. The input signal 63 isapplied to a first channel 60 which has a gain which is chosen inaccordance with a gain characteristic described infra. The low passfilter 62 passes the digital data but rejects the time varying signal ofthe composite signal. The output of the amplifier 64 is applied to asumming amplifier 66. The input signal is also applied to a secondchannel 68 which includes a phase lock loop 70 which is designed toresonate in response to the fundamental frequency of the time varyingsignal of FIG. 3(a). The output of the phase lock loop 70 is applied toan amplifier 72 having a gain chosen in accordance with the gaincharacteristic described infra. The output of amplifier 72 is applied toa detector 74 which rectifies the output of the phase lock loop toproduce a DC level signal which is applied to the summing amplifier 66.The output of the summing amplifier 66 is applied to a thresholddetector 76, which has a Schmitt trigger, produces a signal which isapplied to the one shot multivibrator 39 of FIGS. 1 and 2. The timevarying signal generator will have n-1 additional channels where n isthe number of different frequency tones which are used in the compositesignals 24 of FIG. 3(c). Each additional channel is of identical designto channel 2 except that the phase lock loop 70 of each additionalchannel is designed to respond to a different tone of the frequenciespresent in the signal of FIG. 3(c).

The ratio of the gains of the amplifiers 64 and 72 is chosen inaccordance with the following relationship: ##EQU1## wherein m is themodulation depth of the time varying signal with respect to theamplitude of the non-zero level and N₂, N₁, are the noise levels of theappropriate channels. When the gains of the respective channels arechosen in accordance with the foregoing ratio, the probability ofdetection error is reduced. When more than one frequency is used in thetime varying signal, the gain of the additional channels may be chosenin accordance with the foregoing relationship with the substitution ofthe appropriate quantities for each additional channel.

The signal of FIG. 3(d) may be detected by a time varying signaldetector 34 which is a digital computer that has been programmed in astandard way to detect a bit stream, which detection is initiated by theleading edge of the PCM signal.

ADDITIONAL EMBODIMENTS

While the preferred embodiments of the present invention are fiber opticdigital communication systems, the invention may be used in non-opticalfiber optic communication mediums such as microwave. To use a microwavecommunication medium, a source of microwaves and a suitable modulatormust be provided at the transmitter and at the receiver a suitabledetector of microwaves and a detector of the time varying signal must beprovided.

The present invention is not limited to any particular form ofmodulating the optical carrier wave. Thus while the optical modulatorsof the embodiments of FIGS. 1 and 2 use intensity modulation, otherforms of modulation could be used such as polarizing the optical carriersignal in accordance with the variation of the composite signal withoutdeparting from the spirit of the invention.

The invention is not limited to the transmission of any particularformat of digital data.

While the preferred form of the optical transmission medium that hasbeen discussed in the embodiments of FIGS. 1 and 2 is fiber optics, itshould be understood that the invention may use other opticaltransmission mediums.

I claim:
 1. A transmitter for use in a digital data transmitting systemhaving a transmitter, an optical transmission medium and a receivercomprising:(a) means for producing a time varying signal which is timecoincident with the non-zero amplitude levels of a digital signal havingzero and non-zero amplitudes, the time varying signal being used toencode the occurrence of the non-zero digital amplitudes of the digitalsignal and other information; (b) means for combining two input signalsinto a time coincident composite output signal, the first input signalbeing from a source of a digital signal which is to be transmitted, andthe second input signal being from the means for producing the timevarying signal; and (c) means coupled to the means for combining forproducing an optical signal varying in amplitude in accordance with thecomposite signal.
 2. A transmitter for use in a digital datatransmitting system having a transmitter, an optical transmission mediumand a receiver comprising:(a) means for producing a time varying signalwhich is time coincident with non-zero amplitude levels of a digitalsignal having zero and non-zero levels, the time varying signal beingused to encode at least the occurrence of the non-zero digitalamplitudes of the digital signal; (b) means for combing two inputsignals into a composite output signal, the first input signal beingfrom a source of a digital signal which is to be transmitted and thesecond input being coupled to the means for producing the time varyingsignal; (c) means for producing a beam of an optical carrier signalwhich is adapted to be coupled to the optical transmission medium usedfor transmitting the digital signal; and (d) means disposed within thebeam which is responsive to the means for combining for modulating thebeam with the composite signal.
 3. A transmitter in accordance withclaims 1 or 2 wherein the means for combining comprises an amplifier. 4.A transmitter in accordance with claim 1 or 2 wherein the time varyingsignal is a time varying signal of a fixed frequency which is presentthroughout the entire duration of each non-zero amplitude level of thedigital signal to be transmitted.
 5. A transmitter in accordance withclaim 1 wherein the time varying signal is a digital signal.
 6. Atransmitter in accordance with claim 1 wherein the means for producingthe optical signal is a light emitting diode.
 7. A transmitter inaccordance with claims 1 or 2 wherein the means for producing the timevarying signal comprises means for producing electrical oscillationshaving an input which is adapted to be coupled to the source of digitaldata and an output coupled to the means for combining for producingelectrical oscillations in response to a digital signal of a non-zeroamplitude being present at the input.
 8. A transmitter in accordancewith claim 2 wherein the means for modulating is an intensity modulator.9. A transmitter in accordance with claim 8 wherein the intensitymodulator is a BRAGG cell.
 10. A transmitter in accordance with claim 8wherein the intensity modulator is a POCKELS cell.
 11. A transmitter inaccordance with claim 8 wherein the intensity modulator is a STARK cell.12. A transmitter in accordance with claim 2 wherein the modulatorvaries the polarization of the coherent electromagnetic energy inaccordance with the amplitude of the composite signal.
 13. A transmitterin accordance with claim 2 wherein the means for producing a beam oflight is a continuous wave laser.
 14. A transmitter for use in a digitaldata transmitting system having a transmitter, a transmission medium anda receiver comprising:(a) means for producing a time varying signalwhich is time coincident with non-zero amplitude levels of a digitalsignal having zero and non-zero levels, the time varying signal beingused to encode the occurrence of the non-zero digital amplitudes of thedigital signal and other information; (b) means for combining two inputsignals into a time coincident composite output signal, the first inputsignal being from a source of a digital signal and the second inputbeing coupled to the means for producing the time varying signal; and(c) means coupled to the means for combining for modulating a carriersignal in accordance with the composite signal.
 15. A digital datatransmitting system comprising:(a) means for producing a time varyingsignal which is time coincident with non-zero amplitude levels of adigital signal having zero and non-zero amplitude levels, the timevarying signal being used to encode the occurrence of the non-zerodigital amplitudes of the digital signal and other information; (b)means for combining two input signals into a time coincident compositeoutput signal, the first input signal being from a source of a digitalsignal and the second input being coupled to the means for producing thetime varying signal; (c) means coupled to the means for combining forproducing a modulated optical carrier signal varying in amplitude inaccordance with the composite signal; (d) an optical transmission mediumhaving an input and an output, the input being coupled to the means forproducing a modulated optical carrier signal; (e) means for detectingthe modulated optical signal to produce a time varying signal whichvaries in accordance with the composite signal, the means for detectingbeing coupled to the output of the optical transmission medium; and (f)means for detecting the time varying signal, the detection of the timevarying signal indicating reception of a digital signal of non-zeroamplitude and other information.
 16. A digital data transmitting systemin accordance with claim 15 further comprising:(a) means responsive tothe means for detecting the time varying signal for producing a pluseeach time the presence of a time varying signal is detected to reproducethe transmitted digital signal; and (b) means for detecting the otherinformation contained within the time varying signal in addition to thepresence of a non-zero amplitude of the digital signal in response toeach time the means for detecting detects the presence of a time varyingsignal.
 17. A digital data transmitting system comprising:(a) means forproducing a time varying signal which is time coincident with non-zeroamplitude levels of a digital signal having zero and non-zero levels,the time varying signal being used to encode the occurrence of thenon-zero digital amplitudes of the digital signal and other information;and (b) means for combining two input signals into a time coincidentcomposite output signal, the first input signal being from a source of adigital signal and the second input signal being coupled to the meansfor producing the time varying signal; (c) means for producing a beam ofan optical carrier signal which is used for transmitting the digitalsignal; (d) means disposed within the beam which is responsive to themeans for combining for modulating the beam of coherent electromagneticenergy with the composite signal; (e) an optical transmission mediumhaving an input and an output, the input being coupled to the means forproducing a modulated optical carrier signal; (f) means for detectingthe modulated optical carrier signal to produce a time varying signalwhich varies in accordance with the composite signal, the means fordetecting being coupled to the output of the optical transmissionmedium; and (g) means for detecting the time varying signal, thedetection of the time varying signal indicating the reception of adigital signal of a non-zero amplitude and the other information.
 18. Adigital data transmitting system in accordance with claim 17 furthercomprising:(a) means responsive to the means for detecting the timevarying signal for producing a pulse each time the presence of a timevarying signal is detected to reproduce the transitted digital signal;and (b) means for detecting the other information contained within thetime varying signal in addition to the presence of a non-zero amplitudeof the digital signal in response to each time the means for detectingdetects the presence of a time varying signal.
 19. The digitaltransmitting system in accordance with claim 17 wherein:(a) the meansfor producing the pulse comprises a one-shot multivibrator and (b) ameans is coupled between the means for detecting the modulated opticalsignal and the means for detecting the time varying signal foramplifying the time varying signal produced by the detection of themodulated optical signal.
 20. A system in accordance with claim 15 or 17wherein the means for combining comprises an amplifier.
 21. A system inaccordance with claim 20 wherein the time varying signal is a signal ofa fixed frequency which is present throughout the entire duration ofeach non-zero amplitude level of the digital signal to be transmitted.22. A system in accordance with claim 20 wherein the time varying signalis a digital signal.
 23. A system in accordance with claim 20 whereinthe time varying signal is a series of time varying signals of a singlefrequency which are separated by zero amplitude levels.
 24. A system inaccordance with claim 20 wherein the means for producing the opticalsignal is a light emitting diode.
 25. A system in accordance with claims15 and 17 wherein the means for producing the time varying signalcomprises means for producing electrical oscillations having an inputwhich is adapted to be coupled to the source of digital data and anoutput coupled to the means for combining for producing electricaloscillations in response to a digital signal of a non-zero amplitudebeing present at the input.
 26. A system in accordance with claim 18wherein the means for modulating is an intensity modulator.
 27. Atransmitter in accordance with claim 26 wherein the intensity modulatoris a STARK cell.
 28. A transmitter in accordance with claim 26 whereinthe intensity modulator is a BRAGG cell.
 29. A transmitter in accordancewith claim 26 wherein the intensity modulator is a POCKELS cell.
 30. Areceiver for use in a digital data transmission system having atransmitter, an optical transmission medium and a receivercomprising:(a) means for detecting a composite signal which has beenmodulated upon a received optical carrier signal, the composite signalcomprising a time coincident non-zero amplitude digital signal and atime varying signal which encodes the non-zero digital amplitude of thedigital signal and other information; and (b) means for producing apulse in response to the detection of each time varying signal.
 31. Areceiver in accordance with claim 30 wherein the means for detecting thecomposite signal comprises:(a) a first channel coupled to the compositesignal having low pass filter means for passing the digital signal butrejecting the time varying signal; (b) a second channel coupled to thecomposite signal having means responsive to the time varying signal toproduce an output signal in response to the time varying signal; (c)summing means coupled to the low pass filter means and to the outputsignal of the means responsive to the time varying signal for adding thedigital signal and the output signal; and (d) threshold detection meansfor detecting the sum of the output signal and the digital signal as thedigital signal.
 32. A receiver in accordance with claim 31 wherein themeans responsive to the time varying signal includes a phase lock loop.33. A receiver in accordance with claim 31 wherein:(a) a first amplifieris contained in the first channel with a first gain; (b) a secondamplifier is contained in the second channel with a second gain; and (c)the ratio of the gains is ##EQU2## wherein m is the modulation depth ofthe time varying signal with respect to the amplitude of the non-zerolevel and N₁ and N₂ are the noise levels of the appropriate channels.34. A receiver in accordance with claim 32 wherein:(a) a first amplifieris contained in the first channel with a first gain; (b) a secondamplifier is contained in the second channel with a second gain; and (c)the ratio of the gains is ##EQU3## wherein m is the modulation depth ofthe time varying signal with respect to the amplitude of the non-zerolevel and N₁ and N₂ are the noise levels of the appropriate channels.35. A receiver in accordance with claim 32 further comprising:meanslocated in the second channel which is coupled to the phase lock loopfor detecting an output signal of the phase lock loop to produce theoutput signal in response to the time varying signal.
 36. A receiver inaccordance with claim 35 further comprising means for detecting otherinformation which is coupled to the means located in the second channelwhich is coupled to the phase lock loop.